Understanding how species-specific traits and density-dependent competition modulate the effects of extreme climate events is essential for informing future forest management strategies. We analyzed the radial growth of western larch (Larix occidentalis) and Douglas-fir (Pseudotsuga menziesii var. glauca) trees growing in unmanaged old-growth stands - and western larch grown at two experimentally controlled densities - at four sites across western Montana, USA. Differences in leaf habit (deciduous vs. evergreen) and divergent water use strategies (anisohydric vs. isohydric) of western larch and Douglas-fir provided a model for linking growth-climate relationships to functional traits. We evaluated resistance, resilience, and recovery of basal area growth to drought and growing season frosts between (a) species and (b) experimental stand densities. Western larch growth exhibited a negative relationship with previous autumn frost (October freezing degree days; FDD), while Douglas-fir showed no response. Radial growth response to spring frost (May-June FDD) was negative for both species, with western larch exhibiting greater sensitivity. Both species showed reduced growth in years following drought conditions (previous July-August Palmer Drought Severity Index; PDSI), while only Douglas-fir was sensitive to contemporaneous July-August PDSI. Growth of larch trees in low-density stands was more resistant and resilient to previous summer drought and spring frost than that of trees in high-density stands. Furthermore, observations of greater soil volumetric water content in low-density stands provide a mechanistic link for drought responses. However, growth response to autumn frost was density independent. Our results suggest that early thinning has the potential to limit the negative impacts of summer drought and spring frost in sensitive species, and that species-specific physiological traits can drive divergent growth responses to extreme climate events.

Climate-induced northward advance of boreal forest is expected to lessen albedo, alter carbon stocks, and replace tundra, but where and when this advance will occur remains largely unknown. Using data from 19 sites across 22 degrees of longitude along the tree line of northern Alaska, we show a stronger temporal correlation of tree ring growth with open water uncovered by retreating Arctic sea ice than with air temperature. Spatially, our results suggest that tree growth, recruitment, and range expansion are causally linked to open water through associated warmer temperatures, deeper snowpacks, and improved nutrient availability. We apply a meta-analysis to 82 circumarctic sites, finding that proportionally more tree lines have advanced where proximal to ongoing sea ice loss. Taken together, these findings underpin how and where changing sea ice conditions facilitate high-latitude forest advance.

Unprecedented modern rates of warming are expected to advance boreal forest into Arctic tundra 1 , thereby reducing albedo 2 – 4 , altering carbon cycling 4 and further changing climate 1 – 4 , yet the patterns and processes of this biome shift remain unclear 5 . Climate warming, required for previous boreal advances 6 – 17 , is not sufficient by itself for modern range expansion of conifers forming forest–tundra ecotones 5 , 12 – 15 , 17 – 20 . No high-latitude population of conifers, the dominant North American Arctic treeline taxon, has previously been documented 5 advancing at rates following the last glacial maximum (LGM) 6 – 8 . Here we describe a population of white spruce ( Picea glauca ) advancing at post-LGM rates 7 across an Arctic basin distant from established treelines and provide evidence of mechanisms sustaining the advance. The population doubles each decade, with exponential radial growth in the main stems of individual trees correlating positively with July air temperature. Lateral branches in adults and terminal leaders in large juveniles grow almost twice as fast as those at established treelines. We conclude that surpassing temperature thresholds 1 , 6 – 17 , together with winter winds facilitating long-distance dispersal, deeper snowpack and increased soil nutrient availability promoting recruitment and growth, provides sufficient conditions for boreal forest advance. These observations enable forecast modelling with important insights into the environmental conditions converting tundra into forest.

Nearly a century of fire suppression in most forested land of the United States has limited researchers' ability to construct and rigorously test conceptual models of forest structural development in mixed-conifer ecosystems. As a result, land managers must rely on conceptual models of forest development that may overemphasize idealized stand structures and developmental pathways, which ultimately hampers management of many forest systems for resilience to future climate change impacts. We sought to determine the relative importance of fire history (frequency, severity, and time since fire) and biophysical variables on forest structural diversity and development. Importantly, we conducted our study in an ecosystem with a contemporary active fire regime where wildland fire has been managed as an ecosystem process for four decades. Using data from unburned (>= 80 years since fire), once-burned and twice-burned mixed-conifer forests in the Bob Marshall Wilderness of Northwest Montana, we conducted a hierarchical clustering analysis to identify forest stand structure classes. We then used a Classification and Regression Tree analysis, combined with other post-hoc analyses, to elucidate the biophysical and disturbance history drivers that lead to each structure class. The cluster analysis revealed six forest structure classes. The CART analysis indicated that time since fire plays a large role in determining forest structure, but at intermediate time scales structure is further shaped by repeat fires and interactions with biophysical variables. The CART and posthoc analyses did not, however, indicate a singular fire history or biophysical pathway to any one structure class. We synthesize our results in a conceptual model of forest structural development under an active fire regime. This model supports existing theory that succession following severe fire plays a large role in shaping forest structure. It also recognizes the role of fire at variable severities and frequencies, the physical environment, and tree community composition in influencing forest structural development. The complexity of forest structure and development generated by an active fire regime points to the need to incorporate a process-based view of wildfire if the goal is to manage for improved resiliency and adaptive capacity to future climate change impacts.

Warming-induced mountain pine beetle (Dendroctonus ponderosae; MPB) outbreaks have caused extensive mortality of whitebark pine (Pinus albicaulis; WBP) throughout the species' range. In the highest mountains where WBP occur, they cross alpine treeline ecotones (ATEs) where growth forms transition from trees to shrub-like krummholz, some of which survived recent MPB outbreaks. This observation motivated the hypothesis that ATEs are refugia for WBP because krummholz growth forms escape MPB attack and have the potential to produce viable seed. To test this hypothesis, we surveyed WBP mortality along transects from the ATE edge (locally highest krummholz WBP) downslope into the forest and, to distinguish if survival mechanisms are unique to ATEs, across other forest ecotones (OFEs) from the edge of WBP occurrence into the forest. We replicated this design at 10 randomly selected sites in the U.S. Northern Rocky Mountains. We also surveyed reproduction in a subset of ATE sites. Mortality was nearly absent in upper ATEs (mean ± SE percent dead across all sites of 0.03% ± 0.03% 0-100 m from the edge and 14.1% ± 1.7% 100-500 m from the edge) but was above 20% along OFEs (21.4 ± 5.2% 0-100 m and 32.4 ± 2.7% 100-500 m from the edge). We observed lower reproduction in upper ATEs (16 ± 9.9 cones/ha and 12.9 ± 5.3 viable seeds/cone 0-100 m from the edge) compared to forests below (317.1 ± 64.4 cones/ha and 32.5 ± 2.5 viable seeds/cone 100-500 m from the edge). Uniquely high WBP survival supports the hypothesis that ATEs serve as refugia because krummholz growth forms escape MPB attack. However, low reproduction suggests ATE refugia function over longer time periods. Beyond our WBP system, we propose that plant populations in marginal environments are candidate refugia if distinct phenotypes result in reduced disturbance impacts.

Understanding the key mechanisms that control northern treelines is important to accurately predict biome shifts and terrestrial feedbacks to climate. At a global scale, it has long been observed that elevational and latitudinal treelines occur at similar mean growing season air temperature (GSAT) isotherms, inspiring the growth limitation hypothesis (GLH) that cold GSAT limits aboveground growth of treeline trees, with mean treeline GSAT ~6–7°C. Treelines with mean GSAT warmer than 6–7°C may indicate other limiting factors. Many treelines globally are not advancing despite warming, and other climate variables are rarely considered at broad scales. Our goals were to test whether current boreal treelines in northern Alaska correspond with the GLH isotherm, determine which environmental factors are most predictive of treeline presence, and identify areas beyond the current treeline where advance is most likely. We digitized ~12 400 km of treelines (>26 K points) and computed seasonal climate variables across northern Alaska. We then built a generalized additive model predicting treeline presence to identify key factors determining treeline. Two metrics of mean GSAT at Alaska's northern treelines were consistently warmer than the 6–7°C isotherm (means of 8.5°C and 9.3°C), indicating that direct physiological limitation from low GSAT is unlikely to explain the position of treelines in northern Alaska. Our final model included cumulative growing degree‐days and near‐surface (≤1 m) permafrost probability, which together may represent the importance of soil temperature. Our results indicate that mean GSAT may not be the primary driver of treeline in northern Alaska or that its effect is mediated by other more proximate, and possibly non‐climatic, controls. Our model predicts treeline potential in several areas beyond current treelines, pointing to possible routes of treeline advance if unconstrained by non‐climatic factors.

Wilderness areas offer value to society as a source of scientific information. We used fire perimeter records from the upper South Fork Flathead River watershed (Montana) to characterize the area burned one or more times during three periods: the pre-fire exclusion period (1889-1934), the fire exclusion period (1935-1980), and the fire management period (1981-2017). We also quantified the effects of a recent reburn on forest structure and fuels using a before-after-control-impact study design. Total area burned and area burned multiple times depended strongly on time period. The active fire regime during the fire management period mirrored total area burned and area reburned in the pre-exclusion period. At once-burned sites, fuel loads for most fuel types increased or were stable from 2011 to 2015, reflecting ongoing deposition of fire-killed branches and trees. In contrast, the second fire either reduced or maintained surface fuels in 2015 relative to 2011 levels. Seedlings decreased significantly in the twice-burned plots while there was no change in once-burned plots; live overstory tree densities were stable over time in both once- and twice-burned plots. Managers can use the results presented here to inform the design and monitoring of forest landscape restoration prescriptions.

Understanding the processes that control alpine treelines, the elevational limits of tree growth forms, has been a central question in ecology and is growing in importance with concern over climate change. Cool summer air temperatures are currently thought to be the ultimate limiter of upright tree growth at alpine treelines globally. However, winter damage has long been recognized as a shaping force near alpine treelines. Low-growing krummholz growth forms provide an opportunity to test hypotheses about the controls of upright growth in environments above current treelines. To distinguish between effects of growing season temperature, winter damage and their interaction on preventing upright growth in krummholz, we conducted a field experiment on krummholz growth forms of Pinus albicaulis over the summer and winter of 2015-2016 at 10 mountain top sites in the Tobacco Root Mountains, Montana, USA. We experimentally manipulated four factors using a fully crossed design: shoot position (natural low position in the krummholz mat vs. propped up above the krummholz mat), summer warming (warming chamber vs. ambient), winter exposure (shelter cage vs. exposed), and elevation position (local high vs. low krummholz limits). We also conducted an observational study of the climatic conditions associated with recent natural emergent stem establishment from krummholz. Experimentally propped shoots that were exposed in winter experienced the highest mortality (10%-50%), while propped shoots in shelter cages and shoots located within the krummholz mat, whether caged or not, had low mortality (0%-10%). Summer warming had little influence on shoot mortality. Surviving mat shoots had marginally higher growth rates than surviving propped shoots during the early growing season after treatments were established. Natural emergent stem establishment was associated with warmer than average summer temperatures, but also warmer winter temperatures, lower winter wind speeds, and lower snowpack. Synthesis. Our results suggest winter damage plays a more important role than does growing season temperature in maintaining the krummholz growth form. While warming may increase opportunities for emergent shoot establishment above krummholz mats, establishment of upright trees in the krummholz zone will also require climatic change that reduces wind and snow transport which cause winter damage.

Implementing treatments to create structural complexity and spatial heterogeneity within forest stands can be difficult and time consuming. We asked if real-time implementation monitoring with an Android OS tablet application can facilitate successful implementation of such treatments. We compared two tree-marking methods—free selection (FS) and individuals, clumps and openings (ICO)—which were used to implement the same silvicultural prescription. ICO marking guidelines differed from FS in one way: inclusion of targets describing the number of tree clumps of different sizes to be left, with real-time monitoring of progress towards these targets using the tablet app. ICO trials were more successful at producing desired conditions. FS trials resulted in stand densities below the target and lacked large and very large tree clumps. Implementation efficiency (trees marked per person-hour) was similar between the two systems. Real-time implementation monitoring of quantitative targets can increase the likelihood of treatment success.

Silvicultural thinning treatments to restore whitebark pine (Pima albicaulis) are widely used in subalpine forests throughout the western United States (US) and Canada. The objectives of these treatments are to (1) improve the condition of whitebark pine at all ages, (2) to improve seedling recruitment processes, and (3) mitigate the damage caused by mountain pine beetle (MPB; Dendroctonus ponderosae) and white pine blister rust (WPBR; caused by the fungus Cronartium ribicola). However, there is some disagreement about the ecological basis of restoration and a paucity of information on the effects these activities few treatments have been monitored to assess their success. We investigated the ecological effects of silvicultural restoration treatments in whitebark pine forests and evaluated their success by retrospectively sampling five treatment sites in the western US 6-10 years after implementation. We found strong evidence of growth release at a site previously characterized by closed-canopy stands. Growth responses in more open, park-like stands, however, were variable: we found weak growth increases at one site, weak growth decreases at another and no response at two other sites. At the site with strong growth increases, trees with previous damage from WPBR infection had growth increases similar to uninfected trees. We found low rates of whitebark pine seedling recruitment overall, and no increase in whitebark pine recruitment associated with treatments at any site. However, at one site, treated stands had higher regeneration of non-target species than did untreated stands. Post-treatment mortality (mostly from the late 2000s MPB outbreak) was significantly lower in the treated stand at the closed-canopy site; at the other sites, there was no difference in mortality between treated and untreated stands. The treatments had little detectable effect on short-term growth-climate relationships, although our analyses revealed that whitebark pine growth at our sites was more temperature limited than water limited. While some management goals were achieved, many were not, and there were some unintended consequences. Our results call for a closer examination of the ecological basis of silvicultural restoration treatments in whitebark pine and an expanded use of adaptive management.

Microsites created by live plants and non-living structures can be important for plant establishment where abiotic stress is high. The activity of herbivores and resulting pattern of seedling survival also affects plant distributions. We investigated factors thought to influence survival of bristlecone pine (Pinus longaeva Bailey) seedlings in the White Mountains, California, USA, with two field experiments in which microsite and exposure to herbivory were manipulated. In the microsite experiment, we planted seedlings underneath wooden shade structures, underneath sagebrush plants, and in exposed locations. In the herbivory experiment, we tested the effects of herbivore exclusion in three different habitat types. We planted seedlings in full wire cages (herbivores excluded), in half cages (herbivores allowed while enabling assessment of possible non-herbivore effects of cages on seedling survival), or with no cage (herbivores allowed), and repeated these treatments in three habitats: below, within, and above a bristlecone woodland. Over three growing seasons, seedlings planted under wooden shade structures had higher survival (28.9%) than seedlings growing under sagebrush (10%) or in exposed areas (3.3%). We found a significant treatment by habitat interaction in the herbivory experiment (x(2) = 12.056, P = 0.017), driven by a clear pattern of high herbivory inside the bristlecone woodland, but not above or below it. Our results suggest that biologically-derived microsites (shelter from dead wood and live shrubs), as well as herbivore-mediated density-dependent mortality, are important determinants of bristlecone pine seedling survival. (C) 2015 Elsevier B.V. All rights reserved.